(1) Field of the Invention
This invention relates to an improved system for selecting interface circuits on a priority basis and applies, in particular, to communication controllers. More particularly, the invention relates to means for controlling the transfers of data via adapters on a priority basis.
(2) Description of the Prior Art
Modern communication controllers are devices intended to be used in data teleprocessing systems to control transmissions over the various lines connected thereto as well as the execution of certain teleprocessing functions. In many installations, the teleprocessing network is controlled by one or more central processing units (CPUs) containing a directory of the rules that govern the processing of data sent to or from the terminal stations in the network. The transmission and reception of data via the transmission lines (which may be physical lines or microwave links) is handled by communication controllers operating under the control of the CPU(s). However, in addition to executing transmission commands, the communication controller performs other functions included in the directory, particularly those which are directly associated with standard transmission procedures. As a result, the CPU is relieved of the responsibility of performing such functions and can concentrate on other tasks.
The communication controller is, therefore, an intelligent, relatively complex device. Its intelligence is mainly concentrated in one or more central control units (CCUs) connected to the terminals through line adapters, and to the CPU(s) through channel adapters. Whenever an adapter needs to transfer data, it initiates a request for service and waits for an answer from the CCU for authorization to transmit the data.
All tasks to be processed by the adapters are not of equal urgency and are therefore assigned different priority levels which levels are represented by binary data. Control of the adapters is a relatively simple matter where each adapter processes only tasks that have the same level of priority as it is only necessary in that case to identify those adapters which are making requests for service to determine which adapter will be serviced first when a selection command is received from the CCU.
However, this is not always feasible because the architecture of the teleprocessing systems often requires that tasks with different priority levels be processed by the adapters. In such cases, the adapters may be combined into groups each provided with auto-selection means such that whenever the CPU wants to communicate with the terminals the adapters belonging to a group can interrogate each other to determine which of them is to respond first. However, this auto-selection process is not instantaneous and involves a time delay which is detrimental to the performance of the controller.
It should be noted that, although this specification deals mainly with adapters and communication controllers, the principles of the present invention apply to any set of interface devices or circuits between a central processing unit and components that make requests for services to which different priority levels have been assigned.
To overcome the above difficulties, it has been proposed in U.S. Pat. No. 4,296,463 issued Oct. 20, 1981, and assigned to the same assignee as this application, to perform a preselection operation before the selection proper. The preselection process is carried out in an asynchronous manner quasi-independently of the CCU to enable the adapters belonging to a given group to interrogate each other, with no significant intervention on the part of the CCU, and to designate the adapter that will be serviced first when a selection command is received from the CCU. This scheme works well where the duration of the preselection cycle is relatively short. However, such is not the case where the adapters involved are numerous and/or remote from each other and where provision must be made for control (or interface) circuits to drive the lines interconnecting the adapters and the CCU. If the time interval between the start of the preselection process and the receipt of the selection command is too short for such process to be completed, then the system becomes unstable and cannot work.
It is an object of the present invention to provide a system for asynchronously preselecting adapters to be serviced on a priority basis, which system minimizes the potential instability that may result from the time interval required to perform the preselection operations.